The initial objective of the proposed research is ultrastructural characterization of a murine Leydig cell tumor known to respond to administered gonadotropin with a rapid increase in testosterone production. The electron microscope will be used to classify the cell types present in the tumor and to explore their patterns of intracellular organization. Ultrastructural changes that accompany gonadotropic stimulation of the tumor cells will be defined in relation to their sequence of development. After establishment of the characteristic fine structure of this functionally defined tumor, attention will be directed to utilization of the tumor as an experimental model in studies of the detailed relationship of intracellular structures to steroid hormone production. This phase of the research will involve incubation of tumor cells in media containing gonadotropin in combination with a variety of agents known to interfere with cholesterol metabolism. Aliquots of tumor cells from each incubation vessel will be studied with the electron microscope. The contents of each incubation vessel will be analysed for esterified cholesterol, which presumably represents hormone precursor material, and for testosterone, the secretory end-product. Specific issues to be resolved include 1) the structural manifestation of the hormone precursor pool, 2) the relationship of mitochondrial structure to hormone synthesis, 3) the significance of various structural forms of smooth endoplasmic reticulum in steroid production, and 4) the role of the Golgi complex in steroid hormone secretion. During maintenance of the tumor in our mouse colony, the influence of administered gonadotropin on tumor growth will be investigated. The relationship between uncontrolled growth, which appears to be inhibited by gonadotropin, and functional differentiation, which is enhanced by gonadotropin, requires clarification. It is not yet known which of these factors contributes most to the virulency of this tumor.